I. Field of the Invention
This invention relates to apparatus for solar heating of water or other heat-conducting liquid. More particularly, it relates to such solar heating apparatus which is simple and inexpensive to fabricate and operate and in which the heat collector is in the shape of an upwardly open hemisphere and thus does not need to be moved to different positions to "track" the sun.
II. Description of the Prior Art
Many types of apparatus have been devised in the prior art in which solar energy is used to heat water or other heat-conducting liquid. Such prior art devices have various shortcomings, depending on the principle on which they are based.
One typical design of solar heat collector for liquid includes a heat-absorbing tube arranged in a serpentine configuration in one plane; such a planar-type collector is mounted at an angle facing the sun so that when the pipe has been heated by the sun's rays, liquid flowing therethrough absorbs the heat. The heated liquid is then utilized either directly or by running it through a heat-transfer system whereby its heat is transferred to a second liquid for use, for example, in heating a residence. While planar-type solar heat collectors perform adequately, they generally include long lengths of liquid-conducting pipe which must be properly bent to form the desired circuitous path for the liquid; in addition, in many such collectors, the liquid-conducting pipe is mounted in a substantially rectangular box, insulated against heat loss on all sides but the side facing the sun and covered with a transparent plate on the side facing the sun. Such features tend to make planar-type solar heat collectors expensive and in many cases difficult to fabricate. The principal difficulty with such a collector, however, is that in order to maximize the amount of heat collected thereby, the collector's position must be changed periodically throughout the daylight hours to "track" the sun, so that rays from the sun will strike the transparent plate surface perpendicularly and thereby not be reflected away from the pipe by the plate surface.
Another type of solar heater typical of prior art devices includes a collector utilizing reflectors or lenses so shaped or arranged as to concentrate the rays from the sun on one or a number of liquid-carrying pipes or, in some cases, an oven. Many of these devices must also be moved periodically to track the position of the sun during the day, so that the maximum amount of sunlight is reflected or concentrated on the liquid-carrying tubes or other container. In those collectors which reflect sunlight as a means of concentrating the same, the reflecting surface must be maintained free of dirt, dust and other materials which would impair its reflectivity; consequently, the reflecting-type units, whether or not they are difficult to fabricate, require meticulous maintenance if they are to perform at their optimum level. Collectors in which the sun's rays are concentrated upon a pipe or the like by means of lenses are both difficult to fabricate and often expensive and they, too, require a high degree of care to optimize the amount of heat extracted therefrom.
U.S. Pat. No. 3,254,644 discloses a solar heat cell which comprises a length of liquid-carrying pipe formed into an upwardly-facing concave conical configuration. The pipe, which is of heat-absorbing material, is heated by the sun's rays and transfers its heat to the liquid circulated through it; the conical shape of the arrangement is stated in the patent to present at least a portion of the tube surface perpendicular or tangent to the sun's rays during the daylight hours, thereby lessening the need for changing position of the cell to track the sun. While the device disclosed represents a step in the right direction, it nonetheless still has two disadvantages; first, forming a length of tube or hose into a tight spiral with a precise conical shape is time-consuming and/or expensive, and second, at those times of the day when the sun's rays strike only a small portion of the tube or pipe perpendicularly, the heat absorbed by that portion is not rapidly and efficiently transferred to the remainder of the pipe or tube, so that at such times the efficiency of the unit is extremely low.